Friction stir welding is a relatively new welding process, a summary of the background to which may be found in U.S. Pat. No. 5,460,317 (Thomas et al), U.S. Pat. No. 5,813,592 (Midling et al), WO 93/10935 (The Welding Institute), and WO 95/26254 (Norsk Hydro A.S.), the specifications of which are hereby fully incorporated herein by reference thereto. Generally, the term “friction stir welding” is understood to encompass any method of welding in which a probe of material harder than the work-piece material is caused to move relative to the work-piece to generate frictional heat causing the work-piece in the region of the probe to become plasticised, the probe effectively entering the work-piece. The probe is conventionally caused to rotate about the probe axis and to move along the work-piece along the length of the weld to be formed.
Studies have been made into how the shape of the FSW tool affects the welding process. One such study is described in the papers entitled “Development of Tri-vex friction stir weld tool” by P. A. Colegrove and H. R. Shercliff and published in two parts (Parts 1 & 2) in “Science and Technology of Welding and Joining 2004” (at pages 345-361 of Vol. 9, No. 4). The report describes various tool shapes including a “Tri-flat” tool, a “Tri-flute” tool and a “Tri-vex” tool. The Tri-flat tool is characterised by a cross-section having three flat regions equiangularly arranged about the axis of the tool, the Tri-flute tool has a cross-section that comprises three concavely shaped regions equiangular arranged about the axis of the tool and the Tri-vex tool has a cross-section that comprises three convexly shaped regions equiangular arranged about the axis of the tool. The report compares the loads on the tool necessary to effect FSW welding and concludes that the loads are lower in relation to the Tri-vex tool than the Tri-flute tool. The report describes the use of a Tri-vex tool to weld thin plate (having a thickness of 6.35 mm) using a pin/probe length of 6.2 mm. Experiments subsequently conducted using such a FSW tool having a Tri-vex tool shape and a pin length of 20 mm to weld together workpieces of a thickness of the order of 20 mm have produced weld joints suffering from defects. It has been found that the defect size produced by the Tri-vex tool can be improved by increasing the spindle speed but this results in the joint being subjected to excessive heat, which in turn gives reduced mechanical properties and a large heat affected zone.
It appears therefore that the use of a Tri-vex FSW tool to join thick work-pieces would not be suitable for joining aluminium alloy materials for use in aircraft manufacture as a result of the difficulties in producing joints of sufficiently high quality. Furthermore, the Tri-vex FSW tool is costly and complicated to manufacture as a result of the need to accurately machine three convexly shaped surfaces at three respective positions around the periphery of the tool and then machine threads onto said convexly-shaped surfaces.
It is an aim of the present invention to provide an improved FSW tool. Additionally or alternatively it is an aim of the present invention to provide a FSW tool that is able to produce in thick workpieces welds of a quality suitable for use in the manufacture of aircraft. Additionally or alternatively it is an aim of the present invention to provide a FSW tool that is able to produce welds of a quality equivalent or better than one or more of the FSW tools of the prior art mentioned above, but with reduced loads on the FSW tool during welding and/or without increasing joint heat input when compared to the use of the corresponding prior art FSW tool.